This project was undertaken to study mutagenesis using monospecific antibodies against the sperm specific enzyme, lactate dehydrogenase-X. A monospecific antibody has been prepared from rabbit anti-serum that reacts with rat sperm and not with mouse sperm. Using this antibody in a sandwich fluorescent antibody technique, the frequency of spontaneous mutation of mouse LDH-X in the DBA/2 mouse to "rad kind" LDH-X molecule has been estimated to be 0.4 times 10 to the minus 6th power. The induced mutation frequency shows an upward trend upon treatment of the mice with increasing dosage of procarbazine.